'Breakthrough of the Year' Redraws Evolutionary Picture

In 2009, an international team of researchers published the results of 15 years of study of “Ardi,” the oldest hominid skeleton uncovered to date. The journal Science named the discovery of this 4.4-million-year-old creature its “Breakthrough of the Year 2009,” and scientists from Case Western Reserve University played an integral part in the field research and analysis of the fossils that have redrawn humanity’s evolutionary family tree.

The remains were first discovered in 1994 near the Awash River in Ethiopia. At 4.4 million years old, Ardi, short for her species’ proper name, Ardipithecus ramidus, is 1.2 million years older than the famed Lucy skeleton (an Australopithecus afarensis) discovered 20 years earlier, also in Ethiopia. According to researchers, Ardi stood about 4 feet tall and weighed approximately 110 pounds.
What makes this female hominid so special are the unique attributes that identify her as a possible human ancestor in the midst of transitioning from climbing on all fours to walking upright on two legs.

Ardi is vastly different from the chimpanzees so often thought to be the model of our forebears. These fossils show that human ancestors and apes were already evolving away from each other more than 4 million years ago, according to Scott W. Simpson, PhD, associate professor of anatomy at Case Western Reserve University School of Medicine and research associate at the Cleveland Museum of Natural History.

The Ardi fossils shed light on a previously poorly understood time period in human evolution and also provide new insights into the common ancestor from which human and ape family trees diverged millions of years ago.

Before the discovery of Ardi, researchers assumed that our common ancestor would be more chimpanzee-like. But Ardi’s anatomy is a unique blend of characteristics—some similar to chimps, some similar to monkeys—including a pelvis designed for both climbing and upright bipedal motion, a fully opposable grasping big toe and small canine and incisor teeth unlike the larger ones of modern chimps. These features indicate that the common ancestor looked and acted less like chimpanzees than previously thought.
“It’s completely different from what we predicted and changes not only the way we think about human ancestors, but chimpanzees and monkeys, too,” Simpson says. “This is one of those quantum leaps in understanding.”

Simpson participated in seven years of field research in Ethiopia with the team and coauthored three of the 11 papers on Ardi. Yohannes Haile-Selassie, PhD, curator and head of physical anthropology at the Cleveland Museum of Natural History and an adjunct professor in the departments of anthropology, anatomy and cognitive science at Case Western Reserve, discovered the skeleton. Haile-Selassie and Simpson, along with Bruce Latimer, PhD, former executive director of the Cleveland Museum of Natural History and an adjunct professor of anatomy, anthropology and cognitive science at Case Western Reserve, and Linda Spurlock, director of Human Health at the museum, contributed to seven of the papers.

The Case Western Reserve researchers were part of a team of 47 scientists from nine nations who contributed to the 11 published papers.